The first enzymatic step common to the biosynthesis of the branched chain amino acids (valine, leucine and isoleucine) is catalyzed by acetohydroxyacid synthase (AHAS; also known as acetolactate synthase; E.C.4.1.3.18). The enzyme catalyzes two parallel reactions: condensation of two moles of pyruvate to give rise to acetolactate, and condensation of a mole of pyruvate and a mole of .alpha.-ketobutyrate to yield acetohydroxybutyrate (Bibliography 1,2). This enzyme is inhibited by the end products of the pathway (valine, leucine and isoleucine) and this is one of the known mechanisms of regulation of this pathway in higher plants (3,4).
AHAS is the target site of four classes of structurally unrelated herbicides. These herbicides include the imidazolinones (5), the sulfonylcarboxamides (6), the sulfonylureas (7,8,9), and the triazolopyrimidines (10,11). These findings have stimulated interest in understanding why a single enzyme is inhibited by so many different classes of compounds. These findings have also suggested that there may be other classes of AHAS inhibitors which may be potentially used as herbicides, algaecides or fungicides. Due to the projected economic significance in finding such AHAS inhibitors, there is a great deal of interest in understanding the enzymology and biochemistry of this enzyme.
Three different forms of AHAS have been characterized in Escherichia coli and Salmonella typhimurium. AHAS from these enterobacteria exists as a tetramer of two large and two small subunits (12,13,14). In contrast, AHAS from plants exists in different aggregation states (15,16,17,18) which have different properties. However, it is not known whether AHAS from plants is composed of homologous or heterologous subunits. Since AHAS has been conserved across bacteria, yeast and plants (19), it has been speculated that a small subunit of AHAS may be present in plants.
Purification and characterization of eukaryotic enzyme has been severely hampered by its extreme lability and low abundance (15,20,21,22).